1. Field of the Invention
The present invention relates to fiberboard and a method of densifying the same. More particularly, it is concerned with production of high density, high strength, cellulosic fiberboard, such as one produced from wood fiber.
2. Description of the Prior Art
Structural insulating fiberboard is defind in ASTM D-1554 as a homogeneous panel made from lignocellulosic fibers characterized by an internal bond produced by interfelting of the fibers, and consolidated as a separate stage in manufacture, to a density of more than 10 pounds per cubic foot (pcf) and less than 31 pcf. There is a marked trend in the fiberboard industry to manufacture general purpose insulating fiberboards in specific density ranges for particular uses. However, it is difficult to manufacture specific boards in the higher portion of the density range. Large amounts of heavy fillers must be added, but then, augmenting binders must be added to recapture loss of strength caused by the fillers.
U.S. Pat. No. 2,639,989 discloses the addition of a hydrated lime slurry, of about 10% undissolved lime solids, as filler to the hot fiber pulp being fed to the head box for interfelting to produce a dried board containing approximately 6% lime solids, providing a density increase to 25-28 pcf, compared to an untreated board density of 16-17 pcf. Besides adding weight, this amount of undissolved lime solids in the resulting fiberboard could cause localized skin and eye irritation in handling, particularly if the board becomes wet.